1. Field of the Invention
The present invention relates to a hydraulic lock for a displacer rod drive mechanism (DRDM) and more particularly to an up position hydraulic lock for DRDM of a pressurized water reactor (PWR).
2.State of the Relevant Art
As is well known in the art, conventional PRW's employ a number of control rods which are mounted within the reactor vessel, generally in parallel axial relationship, for axial translational movement in telescoping relationship with the fuel rod assemblies. The control rods contain materials known as poisons, which absorb neutrons and thereby lower the neutron flux level within the core. Adjusting the positions of the control rods relative to the respectively associated fuel rod assemblies thereby controls and regulates the reactivity and correspondingly the power output level of the reactor.
In certain advanced design PWR's, there are employed both reactor control rod clusters (RCC's) and water displacer rod clusters (WDRC's). WDRC's function as a mechanical moderator control, all of the WDRC's being fully inserted into association with the fuel rod assemblies, and thus into the reactor core, when initiating a new fuel cycle. Typically, a fuel cycle is of approximately 18 months, following which the fuel must be replaced. As the excess reactivity level diminishes over the cycle, the WDRC's are progressively, usually in groups, withdrawn from the core so as to enable the reactor to maintain the same reactivity level, even though the reactivity level of the fuel rod assemblies is reducing due to dissipation over time. Conversely, the RCC's are moved, again in axial translation and thus in a telescoping relationship relative to the respectively associated fuel rod assemblies, for control of the reactivity and correspondingly the power output level of the reactor on a continuing basis, for example in response to load demands, in a manner analogous to conventional reactor control operations.
The DRDM and RCC adjustment mechanisms have corresponding control shafts, or drive rods, which extend through the respective head extensions, flow shrouds, and calandria tubes and are connected to the respectively associated spiders mounting the clusters of RCC rods and WDRC rods, and serve to adjust their elevational positions within the inner barrel assembly and, correspondingly, the level to which the rods are lowered into the lower barrel assembly and thus into association with the fuel rod assemblies therein, thereby to control the activity within the core.
As alluded to above, after a period of operation of a nuclear reactor, it is necessary to be able to access and replace the spent fuel assemblies with fresh fuel assemblies. During this operation, it is necessary to remove the reactor vessel closure head. Since the RCC's and WDRC's are attached to their respective drive mechanisms which are mounted on the reactor vessel closure head, it is expedient to disconnect the RCC's and WDRC's from their corresponding drive mechanisms before removing the reactor vessel closure head while maintaining the RCC's and WDRC's in the nuclear reactor core.
The ability of leaving the RCC's and WDRC's in the nuclear reactor core during refueling may be achieved by providing a disconnect mechanism between the various control and displacer rods and the rod drive mechanisms. One such disconnect mechanism is disclosed by Roman et al. in U.S. Pat. No. 4,147,589,commonly assigned herewith. In Roman et al., there is described a control rod assembly for a nuclear reactor having a remotely disengageable coupling between the control rod and the control rod drive shaft. The coupling is actuated by first lowering then raising the drive shaft. The described motion causes axial repositioning of a pin in a grooved rotatable cylinder, each being attached to different parts of the drive shaft which are axially movable relative to each other.
Other hydraulic drive mechanisms with a latching feature are described by Veronesi in U.S. Pat. No. 4,439,054 and Veronesi et al. in U.S. Pat. No. 4,550,941, both commonly assigned herewith. The former uses a drive mechanism and the latter pressurized reactor coolant to raise a flexible spear past a pivoting latching mechanism. The latches use a biasing mechanism and pressure equalization respectively to permit the drive rod to move downwardly into contact with the pivoting latching mechanism. While Roman et al., Veronesi and Veronesi et al. describe mechanisms for disconnecting a drive mechanism from a reactivity control mechanism in a nuclear reactor, there remains a need for an alternative and simpler mechanism for effecting such a disconnection.